I have 10" drums all the way around, if i swapped to discs could i keep my mc. my buddy swapped to discs on his chevelle and kept his old mc and doesn't seem to have any problems.

Everything I've ever read says not to do it. I have no reason to question them, since master cylinders are so cheap.

Nothing will explode if you do this, its just that the calipers may drag a little because the master cylinder for drums keeps a bit of residual pressure in the lines to overcome the springs on the shoes.

you dont need to replace the master cylinder just the proportioning valve

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Unless you want to risk your brakes dragging I suggest replacing the master cylinder. $20 for a remaned is cheap insurance. Or, this would be a perfect time to spend $100 on a new aluminum unit and $40 for the 2 bolt adapter.

The proportioning valve lessens the pressure to the rears, has nothing to do with the residual pressure kept in the front lines.

Holy smokes Pierre, no offense meant, but there is a lot of false information floating around here. The purpose of the residual pressure years ago on Drum brakes was to keep a little pressure in the system to keep the wheel cylinder cups expanded tight against the cylinder bore to keep them from leaking. Now adays and for quite some time the new wheel cylinders and repair kits have included mechanical cup expanders, or a redesigned spring that incorporates the cup expander feature so they no longer need the residual pressure in the system. My 1975-76 Valiants (4 wheel drums) have no resudual pressure valves in the MC that I am aware of. Yes, I have old factory service manals that show the residual valve and how to replace it.
The main difference today between the all-drum MC and the disk-brake MC is the larger chamber on the disk brake MC that connects to the front disk brakes. Reason is that as the disk pads wear, the caliper piston keeps moving out and does not come back so this uses fluid from the larger chamber.

Craig, no offense taken.

Perhaps there is more then one reason, I don't doubt you, I'm not sure where I read that the residual pressure couteracts the springs but I am pretty sure it was a chrysler publication.

I'm also pretty sure that the residual pressure left in the line lessens the stroke necessary to bring the lines up to working pressure as hydraulic oil, though incompressible in the practical sense, does compress perceptibly in a brake system. Forklifts still use these valves today, even with modern wheel cylinders.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.